38 THE JOURNAL OF GEOLOGY. 



Mr. Turner, to protruding hard rocks, it is possible that a con- 

 siderable portion resulted from deformation when the Sierra 

 Nevada was upheaved. For it will be shown later on that since 

 this peneplain was formed by erosion, the Sierra Nevada has 

 been greatly uplifted, and it would be very remarkable indeed if 

 in the upheaval of such an enormous mass as the Sierra Nevada 

 the original plain of its western slope were not warped and 

 broken. 



Platform of the interior region. — The fact that the baselevel 

 plain passes to the eastward from the northern end of the Sacra- 

 mento valley beneath the lavas of the Lassen Peak district, sug- 

 gests that it may reach the platform of the interior region, which 

 is now covered by volcanic material. Within northeastern Cali- 

 fornia and the adjacent portion of Oregon there are vast stretches 

 of level plains which are nearly of the same altitude above the 

 sea. As far as known, all the surrounding hills and mountains 

 are of lava. There are no projecting peaks of older rocks, and 

 their absence from wide stretches of plateau country tends to 

 show a general level of the subjacent surface analogous to that 

 of the interior plateau in British Columbia described by Dr. G. 

 M. Dawson. 



The erosion plains we have traced upon the borders of the 

 Sacramento valley, in the Klamath Mountains, upon the western 

 slope of the Sierra Nevada, and probably also in the interior 

 region of northeastern California, join one another in such a way 

 as to show that they are simply different portions of one exten- 

 sive baselevel of erosion which formerly spread over a large part, 

 if not the whole, of middle and northern California and the 

 adjacent portion of Oregon. What is the geological age of this 

 plain of erosion ? 



DEPOSITS UPON THE BORDER OF THE ANCIENT BASELEVEL. 



General statement.- — In order to determine the conditions 

 under which the baselevel was developed, and its age, it is 

 necessary to study the formations deposited during its develop- 

 ment. At the eastern edge of the baselevel, in the Sacramento 

 valley, there are three formations, all of which were more or less 



